Do you have the expertise to review an article?
Do you know the field well enough to be able to assess the scientific rigour, novelty, quality and importance of the research?
If the answer to any of these questions is ‘no’, then you should decline the task and tell the journal that this is not your area of expertise (in which case, if you know of another expert in the field, then please suggest them to us when you decline). If you are an expert in only part of the paper, you can still write a report and send it to the journal. However, you must make it clear which parts you are not able to assess.